In the heat treatment of fine-grained material, particularly in the production of cement, and to relieve the rotary kiln as far as possible of the responsibility of supplying the heat required for calcining the material and, hence, to enable it to be made smaller in size, it is known that the rotary kiln may be preceded by a precalcination zone in which the material already preheated in several stages of the preheater is further heated (and hence largely calcined) before it subsequently enters the rotary kiln. The precalcination zone has to be supplied with fuel and with a combustion medium of sufficient oxygen content to support combustion.
It is known that oxygen-containing kiln exhaust gases, a cooling air stream run off from the cooler of the installation, or a mixture of kiln exhaust gases and cooler air may be delivered as a gaseous combustion medium to the precalcination zone. The particular difficulty of this method of precalcination lies in quickly transferring the heat content of the additional fuel to the material as completely and as uniformly as possible.
In one known installation (cf. "Zement-Kalk-Gips" 1970, 250), the fine-grained material being preheated in several stages of the cyclone preheater, additional fuel and a cooling air stream run off from the cooler are introduced into the gas pipe which connects the rotary kiln to the cyclone preheater and which carries the kiln exhaust gases. Hence, this gas pipe forms the actual precalcination zone. In order to obtain an adequate transfer of heat from the fuel to the gas in this precalcination zone in spite of the non-optimal admixture of fuel, material, cooler air, and kiln exhaust, the gas pipe in question has to be made relatively long which increases the space occupied by the installation and adds to the construction costs.
In another known process (German Offenlegungsschrift No. 2,506,580), the preheated material, the additional fuel, and the cooling air stream run off from the cooler are delivered to an auxiliary furnace in which the additional fuel is burnt and the material precalcined. From this auxiliary furnace the material and gas then enter the gas pipe through which the exhaust gases of the rotary kiln flow and which leads to the lowermost stage of the cyclone preheater. The main disadvantages of this known process lie in the considerable outlay involved in providing an additional auxiliary furnace as the actual precalcination zone.
The objects of the present invention are to provide processes and apparatus of the type described above in which, despite minimal outlay on plant for the precalcination zone, heat is transferred as completely as possible and, in particular, as uniformly as possible from the fuel to the material.